In Spain que already have that.

There is an app that let you show only some data of your national ID and it’s verifiable by the other party. One of their usecases is specifically age verification. The app shows that the user is over 18 or under 18 and the portrait. There is a qr that can be used for instant verification.

But signing is kind of different. For signing you need an unique id.

I signed this petition with the digital certificate. It really never asked me for much more data. But the digital certificate includes full name, country and national ID number. But I suppose that’s the minimum needed for a valid signature.